Since their discovery single-walled carbon nanotubes (SWNTs) have generated much interest as one of the best candidates for electronic devices owing to their exceptional conductivity and field-effect transistor (FET) behaviour. The undesired presence of metallic nanotubes along with semiconducting nanotubes, however, is the major hurdle to obtain ideal semiconducting devices, i.e. an unsolved obstacle to the realization of the widespread applications of carbon nanotubes is the control of nanotube electronic properties.
During nanotube growth processes, the wrapping around and joining of a graphene sheet leads to many possible chiralities. With most growth processes, about a third of the nanotube species are metallic (for example, met-SWNTs) and the rest semiconducting (for example, sem-SWNTs).
For use as the FET active material, only sem-SWNTs are desired. The poor growth selectivity of sem-SWNTs and efficiency of destroying met-SWNTs make efficient post-synthesis separation schemes necessary. Various approaches have been attempted to isolate semiconducting SWNTs from metallic ones for enhancing the semiconducting characteristics of devices, including dielectrophoresis, selective oxidation, aromatic extraction, surfactant extraction, amine extraction, surface alignment, removal of nanotube bundles, selective polymer wrapping, density gradient centrifugation and selective chemical functionalization. These methods, however, are not able to produce high quality SWNT-nets with high yield or involve complicated and costly procedures. Recent studies based on density gradient ultracentrifugation (DGU) and gel-based separation techniques have demonstrated promising results for sorting SWNTs. And notably, a very recent report showed that metallic conductivity of SWNT-nets can be effectively suppressed by cycloadditive reaction (Kanungo et al., Science 2009, 323, 234-237). This method, however, relies on harsh and lengthy reaction processes. And the yield-efficiency of high performance SWNT-net from this method was not reported.
Thus it is an object of the present invention to provide alternative methods for isolating different species of carbon nanotubes.